Energy intake and dietary macronutrient composition is the most difficult part of the energy balance equation to measure. In a natural history study of factors which predict food intake and using an inpatient computerized vending machine system, food intake has been measured over 180 individuals. In individuals undergoing the study more than once the intra-class correlation coefficient is very high (r=0.9) indicating that these studies although performed in an inpatient setting are very reproducible. This study has demonstrated that higher respiratory quotient, the ratio of carbohydrate oxidation to lipid oxidation which predicts weight gain, also predicts weight gain. The component of respiratory quotient which best predicted food intake was higher carbohydrate oxidation indicating a role for carbohydrate (likely glycogen) balance in the regulation of short term food intake. However, we did not find that glycogen depletion increased food intake in a follow-up study. As noted above respiratory quotient is a ratio of carbohydrate to lipid oxidation. Central hypothalamic long chain fatty acid signaling can affect both food intake and peripheral lipid oxidation. Thus, we are planning future studies which investigate the role of these fatty acids in controlling food intake. In particular, we are developing a study which investigates nasal administration of a long chain fatty acid with subsequent measurement of food intake. We have also demonstrated that one of the most important determinants of food intake is fat free mass. There is a strong positive association between fat free mass (adjusted for height) and food intake, which persists even when adjusted for fat mass. Fat free mass is the major determinant of energy expenditure, and 24 hour energy expenditure is also strongly associated with energy intake. Furthermore, even after adjustment for fat free mass, the residual of energy expenditure was still associated with energy intake. This indicates homeostatic sensing of energy needs is determined primarily by metabolic rate which is driven largely by fat free mass. To develop methods to measure free living food intake, we have ongoing studies which are validating chewing and swallowing monitors using our automated vending machines. Our initial pilot study of ten individuals demonstrated some positive results, but we are continuing with an upgraded chewing monitor to better capture eating episodes. We are continuing our study investigating stable isotopes as biomarkers for dietary patterns. Individuals (n=40) will consume diets over 3 months which vary by meat, soda and fish content and plasma, hair, and adipose tissue samples will be analyzed for changes in the stable isotopes C13 and N15. We will also investigate how these dietary patterns affect behavioral and performance testing, glucose tolerance and energy expenditure. We have currently completed 13 volunteers in this intensive study which requires a 14 week stay on our inpatient unit.